This invention relates to a fire detector, and in particular to a flame detector that detects the presence of a fire by sensing the presence of a flame.
A flame detector is often installed outdoors in a harsh environment. The sensor of the flame detector looks out through a window that can become dirty, and this tends to reduce the effectiveness of the flame detector. The dirt can blow onto the window of the flame detector either as wet or dry air-born particles. There is, therefore, a need to check that the window of a flame detector is clean enough to provide accurate flame detection, and to identify and report any condition where the window is too dirty for the detector to operate correctly.
Traditionally, the window of a flame detector is checked by shining a source provided within the detector through the window onto an external reflector, which reflects that signal back onto a single sensor mounted within the flame detector. The reflected signal is measured, and a reference signal level is taken for a clean window. If the window becomes dirty, then the amplitude of the reflected signal level will be reduced. If the reflected signal level falls below a predetermined reference level, the window is considered to be too dirty for the flame detector to operate correctly.
Known flame detectors incorporate a precise optical system whose optical path gives an accurate, reliable and consistent measurement of the reference signal level. In particular, the external reflector needs to be carefully shaped, and rigidly mounted. The external reflector may also need to be tuned to the requirements of the particular flame detector concerned. An aim of the invention is to provide a flame detector having an inbuilt optical path monitoring facility for checking the cleanliness of its window.